SQLite

          rc = getOverflowPage(pBt, nextPage, 0, &nextPage);
        }
        offset -= ovflSize;
      }else{
        /* Need to read this page properly. It contains some of the
        ** range of data that is being read (eOp==0) or written (eOp!=0).
        */



        int a = amt;
        if( a + offset > ovflSize ){
          a = ovflSize - offset;
        }

#ifdef SQLITE_DIRECT_OVERFLOW_READ
        /* If all the following are true:
        **
        **   1) this is a read operation, and 
        **   2) data is required from the start of this overflow page, and
        **   3) there are no dirty pages in the page-cache
        **   4) the database is file-backed, and
        **   5) the page is not in the WAL file
        **   6) at least 4 bytes have already been read into the output buffer 
        **
        ** then data can be read directly from the database file into the
        ** output buffer, bypassing the page-cache altogether. This speeds
        ** up loading large records that span many overflow pages.
        */
        if( eOp==0                                             /* (1) */
         && offset==0                                          /* (2) */


         && sqlite3PagerDirectReadOk(pBt->pPager, nextPage)    /* (3,4,5) */
         && &pBuf[-4]>=pBufStart                               /* (6) */
        ){
          sqlite3_file *fd = sqlite3PagerFile(pBt->pPager);
          u8 aSave[4];
          u8 *aWrite = &pBuf[-4];
          assert( aWrite>=pBufStart );                         /* due to (6) */
          memcpy(aSave, aWrite, 4);
          rc = sqlite3OsRead(fd, aWrite, a+4, (i64)pBt->pageSize*(nextPage-1));
          nextPage = get4byte(aWrite);
          memcpy(aWrite, aSave, 4);

**   if( isOpen(pPager->jfd) ){ ...
**
** instead of
**
**   if( pPager->jfd->pMethods ){ ...
*/
#define isOpen(pFd) ((pFd)->pMethods!=0)

#ifdef SQLITE_DIRECT_OVERFLOW_READ
/*
** Return true if page pgno can be read directly from the database file
** by the b-tree layer. This is the case if:
**
**   * the database file is open,
**   * there are no dirty pages in the cache, and
**   * the desired page is not currently in the wal file.
*/
int sqlite3PagerDirectReadOk(Pager *pPager, Pgno pgno){
  if( pPager->fd->pMethods==0 ) return 0;
  if( sqlite3PCacheIsDirty(pPager->pPCache) ) return 0;
#ifndef SQLITE_OMIT_WAL
  if( pPager->pWal ){
    u32 iRead = 0;
    int rc;
    rc = sqlite3WalFindFrame(pPager->pWal, pgno, &iRead);
    return (rc==SQLITE_OK && iRead==0);
  }
#endif
  return 1;
}
#endif

/*
** Return true if this pager uses a write-ahead log to read page pgno.
** Return false if the pager reads pgno directly from the database.
*/
#if !defined(SQLITE_OMIT_WAL) && defined(SQLITE_DIRECT_OVERFLOW_READ)
int sqlite3PagerUseWal(Pager *pPager, Pgno pgno){

  int sqlite3PagerSnapshotRecover(Pager *pPager);
  int sqlite3PagerSnapshotCheck(Pager *pPager, sqlite3_snapshot *pSnapshot);
  void sqlite3PagerSnapshotUnlock(Pager *pPager);
# endif
#else
# define sqlite3PagerUseWal(x,y) 0
#endif

#ifdef SQLITE_DIRECT_OVERFLOW_READ
  int sqlite3PagerDirectReadOk(Pager *pPager, Pgno pgno);
#endif

#ifdef SQLITE_ENABLE_ZIPVFS
  int sqlite3PagerWalFramesize(Pager *pPager);
#endif

/* Functions used to query pager state and configuration. */
u8 sqlite3PagerIsreadonly(Pager*);

int sqlite3PCachePercentDirty(PCache *pCache){
  PgHdr *pDirty;
  int nDirty = 0;
  int nCache = numberOfCachePages(pCache);
  for(pDirty=pCache->pDirty; pDirty; pDirty=pDirty->pDirtyNext) nDirty++;
  return nCache ? (int)(((i64)nDirty * 100) / nCache) : 0;
}

#ifdef SQLITE_DIRECT_OVERFLOW_READ
/* 
** Return true if there are one or more dirty pages in the cache. Else false.
*/
int sqlite3PCacheIsDirty(PCache *pCache){
  return (pCache->pDirty!=0);
}
#endif

#if defined(SQLITE_CHECK_PAGES) || defined(SQLITE_DEBUG)
/*
** For all dirty pages currently in the cache, invoke the specified
** callback. This is only used if the SQLITE_CHECK_PAGES macro is
** defined.
*/

/* Return the header size */
int sqlite3HeaderSizePcache(void);
int sqlite3HeaderSizePcache1(void);

/* Number of dirty pages as a percentage of the configured cache size */
int sqlite3PCachePercentDirty(PCache*);

#ifdef SQLITE_DIRECT_OVERFLOW_READ
int sqlite3PCacheIsDirty(PCache *pCache);
#endif

#endif /* _PCACHE_H_ */

  ROLLBACK;
}

do_execsql_test 1.4 {
  SELECT b=int2str(2) FROM t1
} {1 1 1}

#-------------------------------------------------------------------------
db close
sqlite3 db ""
db func int2str int2str

do_execsql_test 2.0 {
  PRAGMA cache_size = -100;
  CREATE TABLE t1(a INTEGER PRIMARY KEY, b);
  WITH c(x) AS ( VALUES(1) UNION ALL SELECT x+1 FROM c WHERE x<100 ) 
    INSERT INTO t1 SELECT x, int2str(x) FROM c;
}

do_execsql_test 2.1 {
  INSERT INTO t1 VALUES(10001, int2str(1001) || int2str(1001) || int2str(1001));
}

do_execsql_test 2.2 {
  SELECT b FROM t1 WHERE a = 10001;
} "[int2str 1001][int2str 1001][int2str 1001]"

finish_test